Generalized high performance navigation system

ABSTRACT

A generalized high performance navigation system is provided using low earth orbit (LEO) satellites. In one embodiment, a method of performing navigation includes receiving a LEO signal from a LEO satellite. The method also includes decoding a navigation signal from the LEO signal. The method further includes receiving first and second ranging signals from first and second ranging sources, respectively. In addition, the method includes determining calibration information associated with the first and second ranging sources. The method also includes calculating a position using the navigation signal, the first and second ranging signals, and the calibration information.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/801,764 filed on May 18, 2006 and entitled“Generalized high performance, low-cost navigation system” which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to navigation and, moreparticularly, to satellite-based navigation techniques.

BACKGROUND

Performance of a navigation system can be determined by the errordistribution in navigation measurements (e.g., accuracy) provided by thesystem. System performance may also depend on its ability to providetimely warnings to users when it should not be used (e.g., integrity).Performance may also be measured by how long a navigation system takesto achieve its first position fix from a cold start (e.g., time to firstfix). In addition, system performance may depend on the fraction of timeor particular circumstances in which specified performance parametersfall within specified limits (e.g., availability).

Unfortunately, the navigation signals provided by various existingnavigation systems often do not provide satisfactory system performance.In particular, the signal power, bandwidth, and geometrical leverage ofsuch navigation signals are generally insufficient to meet the needs ofmany demanding usage scenarios.

Existing navigation approaches based, for example, on Global PositioningSystem (GPS) signals often provide insufficient signal power or geometryto readily penetrate buildings or urban canyons. Such signals may alsobe susceptible to jamming in hostile environments, and can prevent theirusage in safety-of-life applications. Other navigation approaches based,for example, on cellular telephone or television signals typically lackvertical navigation information.

SUMMARY

In accordance with one embodiment of the invention, a method ofperforming navigation includes receiving a low earth orbit (LEO) signalfrom a LEO satellite; decoding a navigation signal from the LEO signal;receiving first and second ranging signals from first and second rangingsources, respectively; determining calibration information associatedwith the first and second ranging sources; and calculating a positionusing the navigation signal, the first and second ranging signals, andthe calibration information.

In accordance with another embodiment of the invention, a navigationdevice includes an antenna adapted to receive a LEO signal from a LEOsatellite and receive first and second ranging signals from first andsecond ranging sources, respectively; a receiver processor adapted todownconvert the LEO signal for further processing; and a navigationprocessor adapted to decode a navigation signal from the LEO signal, andadapted to calculate a position of the navigation device using thenavigation signal, the first and second ranging signals, and calibrationinformation associated with the first and second ranging sources.

In accordance with another embodiment of the invention, a navigationdevice includes means for receiving a LEO signal from a LEO satellite;means for decoding a navigation signal from the LEO signal; means forreceiving first and second ranging signals from first and second rangingsources, respectively; means for determining calibration informationassociated with the first and second ranging sources; and means forcalculating a position using the navigation signal, the first and secondranging signals, and the calibration information.

In accordance with another embodiment of the invention, a method ofproviding a LEO signal from a LEO satellite includes providing aplurality of transmit channels over a plurality of transmit slots,wherein the transmit channels comprise a set of communication channelsand a set of navigation channels; generating a first pseudo random noise(PRN) ranging overlay corresponding to a navigation signal; applying thefirst PRN ranging overlay to a first set of the navigation channels;combining the communication channels and the navigation channels into aLEO signal; and broadcasting the LEO signal from the LEO satellite.

In accordance with another embodiment of the invention, a LEO satelliteincludes an antenna adapted to broadcast a LEO signal from the LEOsatellite; and a processor adapted to: provide a plurality of transmitchannels over a plurality of transmit slots, wherein the transmitchannels comprise a set of communication channels and a set ofnavigation channels, generate a first PRN ranging overlay correspondingto a navigation signal, apply the first PRN ranging overlay to a firstset of the navigation channels, and combine the communication channelsand the navigation channels into the LEO signal.

In accordance with another embodiment of the invention, a LEO satelliteincludes means for providing a plurality of transmit channels over aplurality of transmit slots, wherein the transmit channels comprise aset of communication channels and a set of navigation channels; meansfor generating a first PRN ranging overlay corresponding to a navigationsignal; means for applying the first PRN ranging overlay to a first setof the navigation channels; means for combining the communicationchannels and the navigation channels into a LEO signal; and means forbroadcasting the LEO signal from the LEO satellite.

In accordance with another embodiment of the invention, a method ofproviding a data uplink to a LEO satellite includes determining positioninformation using a LEO signal received from the LEO satellite, a firstranging signal received from a first ranging source, and a secondranging signal received from a second ranging source; determining atiming advance parameter using a local clock reference and a LEOsatellite clock reference; preparing a data uplink signal comprisinguplink data to be broadcast to the LEO satellite; synchronizing the datauplink signal with the LEO satellite using the timing advance parameter;and broadcasting the data uplink signal to the LEO satellite.

In accordance with another embodiment of the invention, a data uplinkdevice includes an antenna adapted to: receive a LEO signal from a LEOsatellite, receive first and second ranging signals from first andsecond ranging sources, respectively, and broadcast a data uplink signalto the LEO satellite; and a processor adapted to: determine positioninformation using the LEO signal, the first ranging signal, and thesecond ranging signal, determine a timing advance parameter using alocal clock reference and a LEO satellite clock reference, prepare thedata uplink signal comprising uplink data to be broadcast to the LEOsatellite, and synchronize the data uplink signal with the LEO satelliteusing the timing advance parameter.

In accordance with another embodiment of the invention, a data uplinkdevice includes means for determining position information using a LEOsignal received from the LEO satellite, a first ranging signal receivedfrom a first ranging source, and a second ranging signal received from asecond ranging source; means for determining a timing advance parameterusing a local clock reference and a LEO satellite clock reference; meansfor preparing a data uplink signal comprising uplink data to bebroadcast to the LEO satellite; means for synchronizing the data uplinksignal with the LEO satellite using the timing advance parameter; andmeans for broadcasting the data uplink signal to the LEO satellite.

In accordance with another embodiment of the invention, a navigationsignal comprises at least a portion of a LEO signal provided by a LEOsatellite, a method of performing localized jamming of the navigationsignal includes filtering a noise source into a plurality of frequencybands to provide a plurality of filtered noise signals in the frequencybands, wherein the navigation signal is spread over a plurality ofchannels of the LEO signal, wherein the channels are distributed overthe frequency bands and a plurality of time slots; generating a PRNsequence corresponding to a modulation sequence used by the LEOsatellite to spread the navigation signal over the channels; modulatingthe filtered noise signals using the PRN sequence to provide a pluralityof modulated noise signals; and broadcasting the modulated noise signalsover an area of operations to provide a plurality of jamming burstscorresponding to the navigation signal, wherein the jamming bursts areconfigured to substantially mask the navigation signal in the area ofoperations.

In accordance with another embodiment of the invention, a navigationsignal comprises at least a portion of a LEO signal provided by a LEOsatellite, a jamming device configured to perform localized jamming ofthe navigation signal includes a noise source adapted to provide a noisesignal; a plurality of filters adapted to filter the noise signal into aplurality of frequency bands to provide a plurality of filtered noisesignals in the frequency bands, wherein the navigation signal is spreadover a plurality of channels of the LEO signal, wherein the channels aredistributed over the frequency bands and a plurality of time slots; aPRN sequence generator adapted to provide a modulation sequence used bythe LEO satellite to spread the navigation signal over the channels; aplurality of oscillators adapted to modulate the filtered noise signalsusing the PRN sequence to provide a plurality of modulated noisesignals; and an antenna adapted to broadcast the modulated noise signalsover an area of operations to provide a plurality of jamming burstscorresponding to the navigation signal, wherein the jamming bursts areconfigured to substantially mask the navigation signal in the area ofoperations.

In accordance with another embodiment of the invention, a navigationsignal comprises at least a portion of a LEO signal provided by a LEOsatellite, a jamming device configured to perform localized jamming ofthe navigation signal includes means for filtering a noise source into aplurality of frequency bands to provide a plurality of filtered noisesignals in the frequency bands, wherein the navigation signal is spreadover a plurality of channels of the LEO signal, wherein the channels aredistributed over the frequency bands and a plurality of time slots;means for generating a PRN sequence corresponding to a modulationsequence used by the LEO satellite to spread the navigation signal overthe channels; means for modulating the filtered noise signals using thegenerated PRN sequence to provide a plurality of modulated noisesignals; and means for broadcasting the modulated noise signals over anarea of operations to provide a plurality of jamming burstscorresponding to the navigation signal, wherein the jamming bursts areconfigured to substantially mask the navigation signal in the area ofoperations.

The scope of the invention is defined by the claims, which areincorporated into this section by reference. A more completeunderstanding of embodiments of the present invention will be affordedto those skilled in the art, as well as a realization of additionaladvantages thereof, by a consideration of the following detaileddescription of one or more embodiments. Reference will be made to theappended sheets of drawings that will first be described briefly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides an overview of an integrated high-performance navigationand communication system in accordance with an embodiment of theinvention.

FIG. 2 provides a further overview of the system of FIG. 1 in accordancewith an embodiment of the invention.

FIG. 3 illustrates an overall operational configuration of the system ofFIG. 1 in accordance with an embodiment of the invention.

FIG. 4 illustrates an approach for implementing low earth orbit signalsin accordance with an embodiment of the invention.

FIG. 5 illustrates an autocorrelation function associated with low earthorbit signals in accordance with an embodiment of the invention.

FIG. 6 illustrates a process of decoding a military navigation componentof a low earth orbit signal in accordance with an embodiment of theinvention.

FIG. 7 illustrates a block diagram of a correlator of a navigationdevice in accordance with an embodiment of the invention.

FIG. 8 illustrates a process of decoding a commercial navigationcomponent of a low earth orbit signal in accordance with an embodimentof the invention.

FIG. 9 illustrates an alternate process of decoding a commercialnavigation component of a low earth orbit signal in accordance with anembodiment of the invention.

FIG. 10 illustrates a process of decoding a civil navigation componentof a low earth orbit signal in accordance with an embodiment of theinvention.

FIG. 11 illustrates a comparison between navigation components of a lowearth orbit signal and GPS codes in accordance with an embodiment of theinvention.

FIG. 12 illustrates a block diagram of a jamming device that may be usedto perform localized jamming of navigation signals in accordance with anembodiment of the invention.

FIG. 13 provides a frequency and time domain representation of theoperation of the jamming device of FIG. 12 in accordance with anembodiment of the invention.

FIG. 14 illustrates a process of generating pseudo random noise inaccordance with an embodiment of the invention.

FIG. 15 illustrates a process of constructing uniformly distributedintegers of a modulo range from a channel selection pool in accordancewith an embodiment of the invention.

FIG. 16 illustrates a process of converting a channel selection pool toa list of random non-overlapping channels in accordance with anembodiment of the invention.

FIG. 17 illustrates a frequency hopping pattern generated by the processof FIG. 16 in accordance with an embodiment of the invention.

FIG. 18 illustrates a block diagram of a receiver processor configuredto receive and sample navigation signals for downconversion inaccordance with an embodiment of the invention.

FIG. 19 illustrates a block diagram of a navigation processor configuredto perform ranging processing in accordance with an embodiment of theinvention.

FIG. 20 illustrates various state variable definitions used by thenavigation processor of FIG. 19 in accordance with an embodiment of theinvention.

FIG. 21 illustrates a block diagram of a tracking module configured toperform signal tracking in accordance with an embodiment of theinvention.

FIGS. 22-29 illustrate various uses of a navigation system to performnavigation in different environments in accordance with variousembodiments of the invention.

FIG. 30 illustrates a generalized frame structure for a low earth orbitsatellite uplink in accordance with an embodiment of the invention.

FIG. 31 illustrates a ground infrastructure to synchronize a low earthorbit satellite data uplink in accordance with an embodiment of theinvention.

FIG. 32 illustrates an implementation of a low level data uplink signalin accordance with an embodiment of the invention.

FIG. 33 illustrates a block diagram of a transmitter to support a lowearth orbit satellite data uplink in accordance with an embodiment ofthe invention.

FIG. 34 illustrates a block diagram of various components of a low earthorbit satellite configured to support a data uplink in accordance withan embodiment of the invention.

Embodiments of the present invention and their advantages are bestunderstood by referring to the detailed description that follows. Itshould be appreciated that like reference numerals are used to identifylike elements illustrated in one or more of the figures.

DETAILED DESCRIPTION

In accordance with various embodiments discussed herein, a navigationsystem employing Low Earth Orbiting (LEO) satellites may be used toimplement various navigation signals to provide high integritynavigation. Passive ranging signals from LEO satellites and othernon-LEO transmitters (e.g., spaceborne and/or terrestrial), may beintegrated into the system.

A reference network of monitor stations may estimate the clock bias,signal structure, and transmitter location or ephemeris of the variousplatforms from which the passive ranging signals are transmitted. Thisestimated information (also referred to as calibration information) maybe conveyed to various navigation devices through a data link with LEOsatellites or other data links.

The navigation devices may be configured to blend the broadcastinformation and the several different types of signals together toperform high-accuracy navigation. The broadcast LEO signal may beimplemented with military, commercial, and civil navigation signals topermit partitioning of users among the different navigation signals andto enable infrastructure cost sharing. An integrated spread spectrum,low probability of intercept and detection (LPI/D) data uplink may alsobe provided as also described herein.

Referring now to the figures wherein the showings are for purposes ofillustrating embodiments of the present invention only, and not forpurposes of limiting the same, FIG. 1 provides an overview of anintegrated high-performance navigation and communication system 100(also referred to as an iGPS system) in accordance with an embodiment ofthe invention. System 100 may include a navigation device 102 (alsoreferred to as user equipment, a user device and/or a user navigationdevice) implemented with appropriate hardware and/or software to receiveand decode signals from a variety of space and terrestrial rangingsources to perform navigation. Such signals may include, for example,satellite broadcasts from GPS, LEO (e.g., Iridium or Globalstar), WideArea Augmentation System (WAAS), European Geostationary NavigationOverlay Service (EGNOS), Multi-functional Satellite Augmentation System(MSAS), Galileo, Quasi-Zenith Satellite System (QZSS), and/or MobileSatellite Ventures (MSV) satellites. Such signals may also includeterrestrial broadcasts from cellular towers, TV towers, WiFi, WiMAX,National Vehicle Infrastructure Integration (VII) nodes, and otherappropriate sources. In one embodiment, navigation device 102 may beimplemented in accordance with various embodiments set forth in U.S.patent application Ser. No. 11/268,317 filed on Nov. 7, 2005 which isincorporated herein by reference

In the example shown in FIG. 1, navigation device 102 may be configuredto receive global positioning system (GPS) signals 106 (e.g., protectedand/or unprotected GPS signals) from conventional navigation satellites.In addition, navigation device 102 may further receive signals 104 fromvarious low earth orbit (LEO) satellites 108. In this regard, each ofLEO signals 104 (also referred to as iGPS signals) may be configured asa composite signal including a communication signal 104A, a militarynavigation signal 104B, a commercial navigation signal 104C, and a civilnavigation signal 104D. Such an implementation allows LEO satellites 108to simultaneously service military, commercial, and civil users, andallows such users to share the costs of operating system 100.

In one example, LEO satellites 108 may be implemented by satellites ofan existing communication system (e.g., Iridium or Globalstar) that havebeen modified and/or reconfigured to support system 100 as describedherein. As also shown in FIG. 1, LEO satellites 108 may be implementedto support crosslink signals 110 between the various LEO satellites 108.

Using GPS signals 106 and/or LEO signals 104, navigation device 102 maycalculate its position (and accordingly the position of an associateduser) to high accuracy. Once determined, the calculated position data(and other data as may be desired) may then be uplinked to LEOsatellites 108 using a spread spectrum data uplink described herein.

Navigation device 102 may be further configured to receive and performnavigation using broadcasts of other space and terrestrial rangingsources as may be desired in particular embodiments. In addition,navigation device 102 may be configured with an inertial measurementunit (IMU) implemented, for example, as a microelectromechanical system(MEMS) device to provide jamming protection as described herein.

Navigation device 102 may be implemented in any desired configuration asmay be appropriate for particular applications. For example, in variousembodiments, navigation device 102 may be implemented as a handheldnavigation device, a vehicle-based navigation device, an aircraft-basednavigation device, or other type of device.

FIG. 2 provides a further overview of system 100 in accordance with anembodiment of the invention. In particular, FIG. 2 illustrates LEOsatellites 108 and GPS satellites 202 in orbit around the earth. FIG. 2further illustrates various aspects of infrastructure subsystems ofsystem 100. For example, system 100 may include a reference network 204configured to receive LEO signals 104 or other ranging signals, GPSground infrastructure 206, and LEO ground infrastructure 208. It will beappreciated that additional spaceborne and/or terrestrial components mayalso be provided in various embodiments of system 100.

FIG. 3 illustrates an overall operational configuration of system 100 inaccordance with an embodiment of the invention. It will be appreciatedthat although a variety of subsystems are illustrated in FIG. 3, all ofsuch subsystems need not be provided in all embodiments of system 100.

As shown in FIG. 3, LEO satellites 108 exhibit rapid angle motionrelative to navigation devices 102 and various illustrated terrestrialsubsystems. Advantageously, this rapid angle motion can aid theterrestrial subsystems in solving for cycle ambiguities. In addition,LEO signals 104 may be implemented with high power relative toconventional navigation signals 106. As such, LEO signals 104 may alsoenable penetration through interference or buildings.

LEO signals 104 may include a ranging and data link to the variousground terminals. As shown in FIG. 3, such terminals may include ageographically diverse reference network 204 and navigation devices 102(illustrated in this example as a cell phone handset, MEMS device, andan automobile).

A variety of satellites are also illustrated, including GPS satellites202, Galileo satellites 306, WAAS satellites 302, and QZSS/MSV 304satellites, any of which may be configured to broadcast ranging and datadownlinks to reference network 204 and navigation devices 102 inaccordance with various embodiments.

It will be appreciated that for purposes of clarity, some rangingsignals are not shown in FIG. 3. For example, in one embodiment, all ofthe illustrated satellites may be configured to broadcast to all ofnavigation devices 102 and reference network 204.

As also shown in FIG. 3, a variety of ranging signals 318 from aplurality of ranging signal sources 310 may be monitored by referencenetwork 204 and navigation devices 102. Reference network 204 may beconfigured to characterize each ranging signal source 310 to providecalibration information associated with each ranging signal source. Suchinformation may be passed to LEO satellite 108 over an appropriate datauplink 320, encoded by LEO satellite 108 into one or more of military,commercial, or navigation signals 104B/104C/104D of LEO signal 104, andbroadcast to navigation devices 102 as part of LEO signal 104. Thecalibration information can then be used by navigation devices 102 tointerpret ranging signals 318 in order to perform navigation incombination with a ranging measurement performed using LEO signal 104.

In general, a variety of transmitters can provide timing and (andtherefore ranging) data. In this regard, for a generalized rangingsource, its associated ranging signal may be received by referencenetwork 204 and navigation devices 102. Reference network 204 maydetermine calibration information associated with the ranging signal,and telemeter such calibration information to navigation devices 102through a data uplink with LEO satellites 108 and/or through terrestriallinks to navigation devices.

For example, FIG. 3, illustrates GPS signals 106 being received by oneof ranging signal sources 310 implemented as a WiFi node. If thecapability to measure the timing (equivalent to range if multiplied bythe speed of light) of pre-defined attributes of a WiFi signal isimplemented within a GPS receiver, the receiver can measure the receivedWiFi and GPS signal times concurrently. The difference between thesequantities can be calculated, time tagged, and transferred to referencenetwork 204 to provide calibration information associated with the WiFinode. Additional calibration information may be determined by referencenetwork 204 in response to receiving GPS signals 106 and other types ofranging signals 318. In each case, reference network 204 may telemeterreal-time calibration information associated with the WiFi node tonavigation devices 102 through LEO satellite 104 over uplink 320 and LEOsignal 104 (e.g., over space-based links). Calibration information mayalso be provided to navigation devices 102 over terrestrial links.Advantageously, each ranging signal source 310 does not necessarily needto be in view of all nodes of reference network 204 if a network 316(e.g., the Internet) is present between the various terrestrial nodes.

As discussed, LEO satellites 108 may be implemented as communicationsatellites (for example, Iridium or Globalstar satellites) that havebeen modified and/or reconfigured as described herein to supportnavigation features of system 100. Tables 1 and 2 below identify variousattributes of Iridium and Globalstar communication satellites,respectively, that may be used as LEO satellites 108 in accordance withvarious embodiments: TABLE 1 Based on GSM Cell Phone Architecture BothFDMA and TDMA 41.667 kHz channel divisions 10.5 MHz downlink allocation40% Root Raised Cosine QPSK modulation at 25,000 sps 90 ms frame TimeSlots: (1) simplex down, (4) 8.28 ms duplex up, (4) 8.28 ms duplex down

TABLE 2 Based on CDMA IS-95 Cell Phone Architecture Both FDMA and CDMA1.25 MHz channel divisions 16.5 MHz downlink allocation Bent-PipeTransponder

In one example where Iridium communication satellites are used toimplement LEO satellites 108, flight computers of the Iridiumcommunication satellites can be reprogrammed with appropriate softwareto facilitate the handling of navigation signals. In another examplewhere Globalstar communication satellites are used to implement LEOsatellites 108, the satellite bent pipe architecture enables groundequipment to be upgraded to enable a variety of new signal formats.

In embodiments where LEO satellites 108 are implemented usingcommunication satellites, the communication satellites may be configuredto support communication signals as well as navigation signals. In thisregard, such navigation signals may be implemented to account forvarious factors such as multipath rejection, ranging accuracy,cross-correlation, resistance to jamming and interference, and security,including selective access, anti-spoofing, and low probability ofinterception.

FIG. 4 illustrates an approach for implementing LEO signals 104 inaccordance with an embodiment of the invention. In particular, blocks410, 420, and 430 of FIG. 4 illustrate the structure of signalstransmitted and received by LEO satellites 108 to provide support forcommunication and navigation signals, where LEO satellites 108 areimplemented using existing Iridium communication satellites. In blocks410, 420, and 430, frequency is shown in the horizontal axis, time isshown in and out of the page, and power spectral density is shown in thevertical axis.

In one embodiment, LEO satellite 108 may be configured to support aplurality of channels implemented as a plurality of transmit slots 402and a plurality of receive slots 404 configured in a time divisionmultiple access (TDMA) fashion over a 90 ms frame width, and furtherconfigured in a frequency division multiple access (FDMA) fashion over a10 MHz frequency bandwidth. In this regard, it will be appreciated thateach channel may correspond to a particular transmit or receive slot ofa frame provided in a particular frequency band. For example, in oneembodiment, LEO satellite 108 may be implemented to support thetransmission of approximately 960 channels, with 240 frequency bandsproviding 4 time slots per frame (e.g., 240 frequency bands×4 timeslots=960 channels).

As shown in block 410, some of the transmit slots 402 and receive slots404 may be associated with existing communications (e.g., shown in FIG.4 as telephone calls 440). The used transmit slots 402 may correspond tothe data provided over communication signal 104A of LEO signal 104transmitted by LEO satellite 108.

It will be appreciated that in the embodiment shown in block 410, aplurality of transmit slots 402 remain unused. In accordance withvarious embodiments of the invention, the unused communication capacityof unused transmit slots 402 may be leveraged to support navigationsignals as described herein.

As shown in block 420, a ranging overlay 422 of pseudo random noise(PRN) may be introduced in each of the remaining unused transmit slots402. Ranging overlay 422 can be run at low average power on achannel-by-channel basis, but with the aggregate ranging overlay 422exhibiting high power to overcome jamming. In contrast, block 430 showsranging overlay 422 implemented using a maximum power spot beam providedby LEO satellite 108.

In one embodiment, ranging overlay 422 may be implemented using acombination of frequency hopping and direct sequence PRN. For thefrequency hopping component, a subset of frequencies may be chosen on apseudo-random basis each burst. Then, within each burst, the data bitsare also chosen on a pseudo-random basis.

In one embodiment, telephone calls 440 may be given priority in transmitslots 402 over ranging overlay 422, with ranging overlay 422 beinglittle affected by occasional missing or corrupted bursts. In anotherembodiment, ranging overlay 422 may be given priority in transmit slots402 over telephone calls 440, with telephone calls 440 similarly beinglittle affected by occasional missing or corrupted bursts.

In one embodiment, ranging overlay 422 may be implemented with as wide abandwidth as possible subject to spectrum regulations. In this case, allavailable channels may be used, and various methods of frequency, time,and code division multiple access (CDMA) may be employed to create adownlink signal that tends to look like flat white noise unless the userknows the code. The flatness provides a signal that is well suited foraccuracy, jam resistance, and multipath rejection. Cross correlation canbe minimized by using an appropriate encryption algorithm made possibleby fast digital signal processing in navigation device 102.

In one embodiment, LEO signal 104 may be implemented as a complex signals(t) versus time t as shown in the following equation:${s(t)} = {A{\sum\limits_{n}{\sum\limits_{m = 1}^{N}{p_{nm}{h\left( {t - {nT}} \right)}{\mathbb{e}}^{j\quad 2\quad\pi\quad{f_{0}{({m - 1})}}{\tau/N}}}}}}$

In the above equation, A is the signal amplitude, n is the symbol index,p is the direct-sequence pseudo-random noise value given as ±1, h is thesymbol impulse response, m is the channel frequency index, f0 is thespread spectrum broadcast span, and N is the number of channelfrequencies forming the spread spectrum broadcast span.

In another embodiment where LEO satellites 108 are implemented byGlobalstar satellites, a low-power direct-sequence code may be providedon each of the 1.25 MHz channels that is orthogonal to telephonytraffic.

FIG. 5 illustrates an autocorrelation function 502 that may beimplemented by navigation device 102 to lock on to LEO signal 104 inaccordance with an embodiment of the invention. In FIG. 5, τ is theautocorrelation argument, R is the autocorrelation function of the basic40% root raised cosine symbol impulse response, N is the number ofchannels allowable by LEO satellite's 108 spectrum allocation (e.g., amaximum of 240 in one embodiment), f₀ is the allowable frequency span(related to N by the channel spacing such that f₀=[41.667 kHz]N in oneembodiment), and φ_(m) is the satellite phase bias for each channel.

In addition, FIG. 5 provides plots 504 and 510 of autocorrelationfunction 502 using different scales. In plot 504, an envelope 506 ofautocorrelation function 502 is shown as being formed by the effectivecorrelation length of the 25 ksps direct sequence data. In thisembodiment, autocorrelation is formed by the aggregation of thebroadband channels separated by 41.667 kHz. For example, for a 10 MHzwide broadcast, the effective direct sequence chip length may be that ofY code, namely 30 m. For comparison, an example GPS coarse/acquisition(C/A) code 512 and an example GPS military (M) code 514 are also shownsuperimposed on plot 510. As shown in plot 510, the side lobes ofautocorrelation function 502 are as readily manageable as those for GPSM-code 514. In this regard, the side lobes of autocorrelation function502 are either highly attenuated or clearly distinguishable.

As previously described, LEO signal 104 may include various navigationsignals including military navigation signal 104B, commercial navigationsignal 104C, and civil navigation signal 104D. As such, navigationdevices 102 may be configured to decode one or more of these signals toperform navigation.

For example, FIG. 6 illustrates a process of decoding militarynavigation signal 104B of LEO signal 104 in accordance with anembodiment of the invention. It will be appreciated that the process ofFIG. 6 may be performed by navigation device 102 in response toreceiving LEO signal 104.

In various applications, it is desirable to implement militarynavigation signal 104B as a high power signal to overcome possiblejamming. Accordingly, as shown in step 1 of FIG. 6, LEO signal 104 mayinclude several parallel channels 602 (shown as 12 channels in FIG. 6)configured to carry military navigation signal 104B. In one embodiment,a pseudo-random process may be used to determine the particular channels602 activated for each broadcast burst from LEO satellites 108. Alsoshown in step of FIG. 6, a string of quadrature phase-shift key (QPSK)symbols 604 are illustrated for each parallel burst on channels 602,with time going into the page. QPSK symbols 604 are modulated with thePRN direct sequence encoding and also exhibit bias and rotation based ontheir frequency offset in LEO signal 104.

In step 2 of FIG. 6, the PRN encoding is despread by rotating each burstto baseband, subtracting off inter-channel bias, and stripping off thePRN direct sequence pattern to provide a set of bursts carrying dataassociated with military navigation signal 104B, as represented bymodified QPSK symbols 606.

In step 3 of FIG. 6, low-bit rate data is demodulated according to a setof M possible orthogonal macro symbols 608. If quarter cycle ambiguitiesfrom the QPSK modulation are present, the combined ambiguities and macrosymbols may not be perfectly orthogonal. Once the data is estimated, ahard decision algorithm strips off the estimated data leaving onlyunmodulated carrier 610.

In step 4 of FIG. 6, the carrier is averaged over the entire burst andthen over each channel. As a result, an in phase and quadraturemeasurement 612 of the instantaneous tracking error can be provided. Aphase locked loop (PLL) of navigation device 102 is then used to trackthe satellite carrier.

FIG. 7 illustrates a block diagram of a correlator of navigation device102 that may be used to perform the process of FIG. 6 in accordance withan embodiment of the invention. A numerically controlled oscillator 702generates a carrier that downconverts the incoming LEO signal 104 (e.g.,received through an antenna of navigation device 102) to a basebandsignal 714. Baseband signal 714 is provided to an upper path 704 thatperforms punctual code carrier tracking. Baseband signal 714 is alsoprovided to a lower path 706 that performs early minus late detection.

In lower path 706, a bank of synthesizers 708 and PRN generators 710replicate each channel of LEO signal 104. In upper path 704, replicatedsignals 712 are mixed with baseband signal 714 to remove all code andphase rotation for each channel separately. A hypothesis generator 716computes the signal associated with each of the possible macro symbols608 and quarter cycle ambiguities, if any. A processor 718 uses amaximum a posteriori (MAP) algorithm to provide a data estimate 720identifying which of the macro symbol hypotheses is most likely. Asshown, data estimate 720 is passed to lower path 706 for use in earlyminus late detection. To perform punctual detection in upper path 704,processor 718 strips off the data and outputs the resulting bursts tosumming block 722 that integrates the aggregate bursts over time toarrive at the in phase and quadrature tracking error 724.

In lower path 706, replicated signals 712 are further modulated by anearly minus late block 726 and a data generator block 728 (using dataestimate 720 received from upper path 704). As shown, the resultingmodulated signals are summed together to form a composite early minuslate replica signal 730 that is mixed with baseband signal 714 and sentto summing block 732 for time averaging to provide an early minus latediscriminator 734. Accordingly, given carrier lock and a sufficientaveraging interval, early minus late discriminator 734 provides ameasure of the instantaneous tracking error.

FIG. 8 illustrates a process of decoding commercial navigation signal104C of LEO signal 104 in accordance with an embodiment of theinvention. It will be appreciated that the process of FIG. 8 may beperformed by navigation device 102 in response to receiving LEO signal104.

As shown, the process of FIG. 8 is similar to the process of FIG. 6,with steps 1-4 of FIG. 8 generally corresponding to steps 1-4 of FIG. 6.However, it will be appreciated that in the process of FIG. 8, fewerchannels 802 (e.g., 2 channels in the illustrated embodiment) are usedin comparison with channels 602 of FIG. 6. Because of the fewer numberof channels 802 used, commercial navigation signal 104C of LEO signal104 may be implemented with lower power and lower bandwidth thanmilitary navigation signal 104B.

FIG. 9 illustrates an alternate process of decoding commercialnavigation signal 104C of LEO signal 104 in accordance with anembodiment of the invention. As shown, the process of FIG. 9 is similarto the process of FIG. 8, with steps 1-2 of FIG. 9 generallycorresponding to steps 1-2 of FIG. 8. However, in step 3 of FIG. 9, itis assumed that downlink data (e.g., calibration information) can bereceived by a navigation device 102 in a manner other than LEO signal104 (for example, from a link to reference network 204 or one or more ofnodes 310 shown in FIG. 3). Further processing can then be performed insteps 4 and 5 of FIG. 9, similar to steps 3 and 4 of FIG. 8,respectively. Advantageously, the insertion of step 3 in the process ofFIG. 9 can provide higher sensitivity in indoor environments. In thisregard, navigation device 102 can receive a reliable representation ofdownlink data from one or more reference stations of reference network204, without requiring navigation device 102 to perform downlink dataand/or quarter cycle stripping, thereby reducing the processing requiredby navigation device 102 and improving signal processing gain.

FIG. 10 illustrates a process of decoding civil navigation signal 104Dof LEO signal 104 in accordance with an embodiment of the invention. Invarious embodiments, the use of civil navigation signal 104D may begenerally focused on carrier-only navigation. As a result, civilnavigation signal 104D may be implemented with relatively narrowbandwidth (for example, approximately 1 MHz) and may be publicly known.As such, channels 1002 used for civil navigation signal 104D may beimplemented without significant spectrum spread. In this regard, it willbe appreciated that channels 1002 illustrated in step 1 of FIG. 10 areclosely grouped in comparison with channels 602 and 802 illustrated instep 1 of each of FIGS. 6, 8, and 9. It will be appreciated that theoperation of steps 1-4 of FIG. 10 will be understood from the operationsteps 1-4 of FIG. 6 previously discussed.

In view of the above discussion, it will be appreciated that in certainembodiments military, commercial, and civil navigation signals 104B,104C, and 104D of LEO signal 104 may be implemented with the followingattributes identified in the following Table 3: TABLE 3 Signal PowerBandwidth Military Maximum Maximum Commercial Moderate High CivilModerate Moderate

In another embodiment of the invention, system 100 can be implemented topermit military use of military navigation signal 104B whilesimultaneously denying use of commercial and/or civil navigation signals104C and 104D to adversaries in a particular area of operations, withoutcompromising use of commercial and civil navigation signals 104C and104D outside the area of operations.

For example, in one embodiment, the decoding of commercial navigationsignal 104C may be conditioned on the use of a distributed encryptionkey that may be permitted to expire over the area of operations. Inanother embodiment, the broadcasting of commercial navigation signal104C by LEO satellites 108 may be selectively interrupted over the areaof operations (for example, individual spot beams from LEO satellites108 may be independently turned off).

In another embodiment, commercial navigation signal 104C and/or civilnavigation signal 104D may be locally jammed within the area ofoperations. In this regard, FIG. 11 illustrates a comparison betweenmilitary navigation signal 104B, civil navigation signal 104D, and GPSC/A code 512, and GPS M-code 514.

As shown in FIG. 11, GPS C/A code 512 can be jammed for militarypurposes by jamming the C/A code band. As also shown in FIG. 11, civilnavigation signal 104D can be viewed as a subset of military navigationsignal 104B in both power spectral density and bandwidth. If rangingoverlay 422 is implemented using both FDMA and TDMA, it can be seen thatcivil navigation signal 104D is manifested in frequency hopping burstsas shown in FIG. 11.

FIG. 12 illustrates a block diagram of a jamming device 1200 that may beused to perform localized jamming of civil and commercial navigationsignals 104C and 104D in accordance with an embodiment of the invention.As shown in FIG. 12, a white noise source 1202 (for example, createdusing Brownian motion) is processed by a filter 1204 to provide a noisesignal 1206 having a bandwidth corresponding approximately to atransmission channel of LEO satellite 108.

A military receiver device 1208, generator 1210, and oscillators1212/1214 are configured to provide multiple channels 1216 correspondingto the instantaneous frequency of civil navigation signal 104D asdetermined by a predefined, published civil PRN sequence. Channels 1216are used to modulate noise signal 1206 which is then upconverted usingadditional illustrated components to emit jamming bursts at preciselythe times, durations, and frequencies of civil navigation signal 104Dreceived from LEO satellites 108 as part of LEO signal 104. It will beappreciated that the above approach can also be used to provide jammingof commercial navigation signal 104C as may be desired in particularimplementations.

FIG. 13 provides a frequency and time domain representation of theoperation of the jamming device of FIG. 12 in accordance with anembodiment of the invention. As shown in FIG. 13, individual noisebursts 1302 provided by jamming device 1200 are focused in a narrowfrequency band 1304 corresponding to civil navigation signal 104D.Advantageously, military navigation signal 104B components (representedby dark rectangles 1306) is effectively unchanged and is fully availablefor military operations.

The generation of ranging overlay 422 at LEO satellite 108 will now bedescribed in relation to FIGS. 14-17. In this regard, various processesdescribed in relation to FIGS. 14-17 may be performed by appropriateprocessors of LEO satellite 108. In addition, LEO satellite 108 may beconfigured with appropriate software and hardware to modulate andbroadcast communication signals (e.g., telephony bursts) in QPSK format.

FIG. 14 illustrates an approach to generating pseudo random noise inaccordance with an embodiment of the invention. The embodiment shown inFIG. 14 uses a counter-based pseudo-random number generator 1400. Inthis regard, a counter value 1402 is combined with a 128-bit encryptiontraffic key 1404 to provide a 128-bit cipher. By associating countervalue 1402 with cipher 1406, the various PRN elements of ranging overlay422 can be constructed. In one embodiment, counter input 1402 and ciphermay each be implemented as 128-bit words using the Advanced EncryptionStandard (AES) process.

As shown in FIG. 14, each counter value 1402 may include a type flag1412 that identifies each counter value 1402 as specifying either achannel selection (e.g., if type flag 1412 is set to a “1”) or directsequence chips (e.g., if type flag 1412 is set to a “0”). If type flag1412 is set to channel selection, then other bits of counter value 1402may specify which channels of a channel selection pool 1408 throughwhich to broadcast data burst chips. If type flag 1412 is set to directsequence, then other bits of counter value 1402 may correspond to a chipblock index 1414 (e.g., specifying a particular one of direct sequencechips 1410 to be broadcast) and a burst count 1416 (e.g., specifying aframe number of the particular direct sequence chip 1410 to bebroadcast).

In one embodiment, cipher 1406 can be used to select a value from achannel selection random number pool 1408 that directs frequencyhopping. In another embodiment, cipher 1406 can be used to select directsequence chips 1410 that fill up the QPSK data bits.

FIG. 15 illustrates a process of constructing uniformly distributedintegers of a modulo range from channel selection pool 1408 inaccordance with an embodiment of the invention. It will be appreciatedthat the process of FIG. 15 may be used in conjunction with channelselection pool 1408 previously described in relation to FIG. 14.

FIG. 16 illustrates a process of converting channel selection pool 1408to a list of random non-overlapping channels in accordance with anembodiment of the invention. The process of FIG. 16 can be used formilitary navigation signal 104B, commercial navigation signal 104C, andcivil navigation signal 104D, by selecting different parameters for Mand N (shown in FIG. 16) in accordance with values provided in thefollowing Table 4: TABLE 4 Signal Power (N) Bandwidth (M) Military Large240 Commercial 1 or 2 >100 Civil 1 or 2 8-32

FIG. 17 illustrates a frequency hopping pattern generated by the processof FIG. 16 in accordance with an embodiment of the invention. As shownin FIG. 16, various random channel selections (associated withcorresponding transmission frequencies) are provided for successivetransmission bursts. It will be appreciated that each frequency and chipis generated in a pseudo random manner using a common key (for example,a 128-bit key) known in advance by LEO satellite 108 and navigationdevice 102.

FIGS. 18-21 illustrate various aspects of navigation device 102 that maybe implemented in accordance with various embodiments of the invention.For example, FIG. 18 illustrates a block diagram of a receiver processor1800 of navigation device 102 configured to receive and sample signalsfor downconversion in accordance with an embodiment of the invention. Asshown in FIG. 18, navigation signals received by an antenna 1802 arefiltered by multi-band filters 1804 (to preselect desired frequencybands), amplified by amplifier 1806, and sampled by sample and holdcircuitry 1808 to provide raw digital RF samples 1816.

Receiver processor 1800 also includes an oscillator 1810 and synthesizer1812 that may be used to synchronize sample and hold circuitry 1808. Invarious embodiments, the sample rate of sample and hold circuitry 1808may be chosen to prevent overlap among aliased, pre-selected frequencybands.

Receiver processor 1800 also includes an IMU 1814 implemented as a3-Axis MEMS gyro and accelerometer having measurement time tagssynchronized to the common clock of the receiver, and may be used toprovide raw digital motion samples 1818. It will be appreciated thatother receiver implementations may alternatively be used to facilitatesingle or multiple-step down conversion.

FIG. 19 illustrates a block diagram of a navigation processor 1900 of anavigation device 102 configured to perform ranging processing inaccordance with an embodiment of the invention. As shown in FIG. 19, aHilbert transform block 1902 converts raw digital RF samples 1816 intocomplex samples 1904. A plurality of tracking modules 1906 are provided.Each tracking module 1906 is associated with a different signal providedin complex samples 1904, and can be used to track either satellite orterrestrial ranging sources.

Navigation processor 1900 provides feed forward commands 1908 totracking modules 1906 based on raw digital motion samples 1818 processedby inertial processor 1916 and extended Kalman filter 1914. Aidinginformation 1908 drives tracking modules 1906 to a small fraction of awavelength. The raw code and carrier phase measurements 1910 fromtracking modules 1906 are read into navigation preprocessor 1912,processed by extended Kalman filter 1914, and combined to provide aposition fix 1918.

FIG. 20 illustrates various state variable definitions employed byextended Kalman filter 1914 of navigation processor 1900 in accordancewith an embodiment of the invention. In one embodiment, a navigationprocessing method disclosed by the previously referenced U.S. patentapplication Ser. No. 11/268,317 may be used to perform navigation usinga plurality of ranging sources.

In FIG. 20, equation 2002 is a model of an integrate and dumpcorrelator. The output tracking error Δy is modeled by averaging overtime T the difference between the actual phase and the phase predictedby the filter. Equation 2004 is a continuous time update model of thecomplete navigation system, including inertial, clock, and all timingand ranging sources, both terrestrial and space based. The estimatorstate vector variables are cumulative correlator phase, user position,velocity, attitude, accelerometer bias, gyro bias, range bias, rangebias rate, clock bias, and clock bias rate. Equation 2006 is the carrierphase observation model, showing time transfer feed forward to the userfrom the reference site taking into account geometry and atmosphericerror.

FIG. 21 illustrates a block diagram of one of tracking modules 1906 inaccordance with an embodiment of the invention. Tracking module 1906receives feed forward commands 1908 to preposition both the code andcarrier phase for the particular ranging signal being tracked bytracking module 1906. Downconverter 1950 rotates the carrier provided incomplex samples 1904 to baseband as a first processing step. Next, thedownconverted signal 1952 signal is split and passed to a matched earlyminus late filter 1954 and a matched punctual filter 1956.

The signal waveform for each ranging signal in view is either pre-storedin user memory or, optionally, refreshed via a data link with a LEOsatellite 108 or a network (e.g., cellular, WiFi, WiMAX, or VII) node.The data link update enables extension of the architecture to be usedwith virtually any transmitted signal. This impulse response (analogousto PRN code for a GPS satellite) forms a basis for matched filterprocessing. The impulse response of a terrestrial signal such ascellular, WiFi, WiMAX, VII, or television may be tailored by retainingthe deterministic portion of the reference signal. Any portion of thesignal that contains non-deterministic characteristics, such as unknowndata, is nulled out in the reference signal. Each of these matchedfilters is then provided with the reference signal structure impulseresponse for implementation in the matched filter/correlator. As aresult, filters 1954 and 1956 provide in-phase and quadraturerepresentations of early minus late tracking errors 1958 and punctualtracking errors 1960, respectively.

Various data structures may be used to encode ranging sources inaccordance with various embodiments of the invention. For example, inone embodiment, a ranging signal can be represented by the followingcode: struct ranging_signal { /* Generalized Ranging Source Parameters*/ impulse_response broadcast_signal; /* signal structure of rangingsource */ double broadcast_frequency; /* ranging source frequency */position broadcast_location; /* phase center of ranging source */ timebroadcast_clock; /* clock bias of ranging source */ };

In the code above, the signal reference waveform is encoded as animpulse response parameter whose time origin is tied to the broadcastclock. The broadcast frequency is the carrier frequency of the rangingsource. The broadcast location is encoded as a precision ephemeris forspace vehicles and as a Cartesian static coordinate for terrestrialranging sources. A clock correction calibrates the ranging sourceagainst system time based on Coordinated Universal Time (UTC) (e.g.,provided by the United States Naval Observatory (USNO)).

In various embodiments, appropriate ground stations may be configured todecipher new ranging signal codes employed by LEO satellites 108 in nearreal-time. In this regard, such ground stations may provide thedeciphered codes to navigation devices 102, thereby permittingnavigation devices 102 to perform navigation using virtually any signal,cooperative or not.

FIGS. 22-29 illustrate various uses of system 100 to perform navigationin different environments services in accordance with variousembodiments of the invention. For example, FIG. 22 illustrates the useof system 100 to provide indoor positioning in accordance with anembodiment of the invention. In this regard, it will be appreciated thatin FIG. 22, navigation device 102 may be positioned inside a building orother structure.

As shown in FIG. 22, navigation device 102 (for example, a handheld usernavigation device) may receive LEO signal 104 either directly from LEOsatellite 108 and additional ranging signals 318 from nodes 310. As alsoshown, reference stations of reference network 204 may also receiveranging signals 318. As previously discussed, reference network 204 maybe configured with appropriate hardware or software to determinecalibration information associated with each ranging signal source 310,passed to LEO satellite 108 over data uplink 320, encoded by LEOsatellite 108 into LEO signal 104, and broadcast to navigation device102 as part of LEO signal 104. The calibration information can then beused by navigation devices 102 to interpret ranging signals 318 in orderto perform navigation in combination with a ranging measurementperformed using LEO signal 104. As a result, navigation device 102 mayutilize LEO signal 104 and ranging signals 318 to perform navigation.

Military navigation signal 104B (e.g., provided by LEO satellite 108 aspart of LEO signal 104) as well as ranging signals 318 (e.g., providedby ranging signal sources 310 such as cellular or television signalsources) may be implemented as high power signals capable of penetratingbuilding materials to reach navigation device 102 when positioned inindoor environments. Accordingly, by using such high power signals inthe approach shown in FIG. 22, navigation device 102 may performnavigation indoors and acquire quickly from a cold start.

FIG. 23 illustrates the use of system 100 to provide indoor positioningin accordance with another embodiment of the invention. It will beappreciated that the implementation shown in FIG. 23 generallycorresponds with the implementation of FIG. 22 previously discussed.However, in the embodiment shown in FIG. 23, navigation device 102 mayalso optionally communicate with reference network 204 or nodes 312 or314 through network 316.

In addition, system 100 may be configured to employ on-tether commercialsignal processing as described herein with regard to FIG. 8. In thiscase, a lower power commercial navigation signal 104C may be used toobtain increased processing gain by transmitting a replica of thenavigation data encoded in commercial navigation signal 104C overranging signals 318. Because the navigation data is removed using theprocess of FIG. 8, tracking loop bandwidth may be significantly reduced.

In one embodiment, navigation device 102 may determine its finalposition fix by forming a vector of pseudoranges for each rangingsource, k, then linearizing about an initial guess for user position, x,and user clock bias τ.$\rho_{k} = {{{{x_{user} - x_{source}}}_{2} + \tau} = {{{{\overset{\_}{x}}_{user} - x_{source}}}_{2} - {{\hat{e}}^{T}\delta\quad x_{user}} + \tau}}$${\delta\quad\rho_{k}} = {{\rho_{k} - {{{\overset{\_}{x}}_{user} - x_{source}}}_{2}} = {{{- {\hat{e}}^{T}}\delta\quad x_{user}} + \tau}}$${\delta\quad\rho} = {\left\lbrack {{- E}\quad\begin{matrix}1 \\1 \\\vdots \\1\end{matrix}} \right\rbrack\begin{bmatrix}{\delta\quad x_{user}} \\\tau\end{bmatrix}}$

The method of least squares is used to refine the user positionestimate: $\min\limits_{\lbrack\begin{matrix}{\delta\quad x_{user}} \\\tau\end{matrix}\rbrack}{{{\delta\quad\rho} - {\left\lbrack {{- E}\quad\begin{matrix}1 \\1 \\\vdots \\1\end{matrix}} \right\rbrack\begin{bmatrix}{\delta\quad x_{user}} \\\tau\end{bmatrix}}}}_{2}$

In another embodiment, system 100 may be implemented to providehigh-accuracy, high-integrity navigation. In this regard, FIG. 24illustrates the use of system 100 to perform navigation using GPSsignals 106 and dual band LEO signals 104 and 104′ in accordance with anembodiment of the invention. Specifically, FIG. 24 shows how asingle-frequency L1 GPS signal may be used with two different LEOsignals 104 and 104′ (e.g., different LEO signals in different frequencybands from different LEO satellites 108 and 108′) to provide a highlevel of navigation performance. In the embodiment shown in FIG. 24, thecarriers of GPS signals 106 and LEO signals 104 and 104′ are sufficientfor navigation—the code phases from the signals need not be used.However, in another embodiment, both code and carrier are used to derivemaximum information from the available observables.

In FIG. 24, stations of reference network 204 may monitor GPS signals106 and LEO signals 104 and 104′, and gather continuous carrier phaseinformation to carry out precise orbit determination of GPS satellites202 and LEO satellites 108. By using different LEO signals 104 and 104′,effects of the ionosphere can be removed, yielding a carrier phasesignal that is ionosphere free. Cycle ambiguities of all GPS satellites202 and LEO satellites 104 and 104′ (e.g., shown by ellipsoids 2402) bycan be estimated by taking advantage of the large angle motion of LEOsatellites 104 and 104′.

The position of navigation device 102 (e.g., an aircraft in thisembodiment) can be determined in FIG. 24 in a manner similarly describedabove with regard to FIGS. 22-23. In particular, the following notationprovides the kth pseudorange measurement to determine the user position,x, at epoch m, and the tropospheric zenith delay, DZ, along with all thesatellite range biases, modeled as continuous variable, b.$\begin{bmatrix}{\delta\quad\rho_{1}} \\\vdots \\{\delta\quad\rho_{M}}\end{bmatrix} = {\begin{bmatrix}\left\lbrack {{- S_{1}}\begin{matrix}1 \\1 \\\vdots \\1\end{matrix}} \right\rbrack & \quad & \quad & \begin{bmatrix}h_{11} \\h_{21} \\\vdots \\h_{K\quad 1}\end{bmatrix} & I \\\quad & ⋰ & \quad & \quad & \vdots \\\quad & \quad & \left\lbrack {{- S_{M}}\begin{matrix}1 \\1 \\\vdots \\1\end{matrix}} \right\rbrack & \begin{bmatrix}h_{1\quad M} \\h_{2\quad M} \\\vdots \\h_{KM}\end{bmatrix} & I\end{bmatrix}\begin{bmatrix}\begin{bmatrix}{\delta\quad x} \\\tau\end{bmatrix}_{1} \\\vdots \\\frac{\begin{bmatrix}{\delta\quad x} \\\tau\end{bmatrix}_{M}}{\frac{D_{Z}}{b}}\end{bmatrix}}$

Again, the method of least squares is used to solve the system ofequations for the position adjustments, time biases, and vector of rangebiases. Even though measurements using GPS signals 106 are singlefrequency and subject to ionospheric bias, the resulting solution doesnot have an ionospheric dependence. Because measurements using LEOsignals 104 and 104′ are ionosphere free and because LEO satellites 104and 104′ exhibit rapid angle motion (compared with the virtually staticmotion of GPS satellites 202), the geometry matrix is full rank with theexception of a common mode between the clock and the ranging biases.This means that the bias estimates for GPS satellites 202 take on valuesthat position the user correctly based on the ionosphere-freemeasurements using LEO signals 104 and 104′.

FIG. 25 illustrates the use of system 100 to perform navigation usingGPS signals 106 and a single LEO signal 104 in accordance with anembodiment of the invention. The orbit geometry of a single LEOsatellite 108 in view tends to place the LEO satellite 108 on atrajectory that aligns a position uncertainty ellipsoid 2502 with thelocal horizontal. In addition to LEO signal 104 and GPS signal 106, athird signal 2504 (e.g., from Galileo satellite 306 or anothersatellite) may be optionally used by navigation device 102 (e.g., anaircraft in this embodiment) to determine its position.

The integrity of a navigation system can be measured by the system'sability to provide timely warnings to users when it should not be used,In this regard, the integrity risk of a navigation system can becharacterized as the probability of an undetected hazardous navigationsystem anomaly. In one embodiment, system 100 can be implemented toprovide high integrity using Receiver Autonomous Integrity Monitoring(RAIM). In RAIM implementations, navigation device 102 can be configuredto monitor measurement self-consistency to detect navigation errorsassociated with a variety of failure modes. Advantageously, the rapidmotion of LEO satellites 108 can facilitate such measurements.

With RAIM, the residual of the least squares fit is used to carry out achi-square hypothesis detection of a system fault. In this regard, thefollowing equation may be used:R=|Δφ−H{circumflex over (x)}|

In the above equation, φ corresponds to ranging measurements, Hcorresponds to a satellite geometry matrix, and {circumflex over (x)}corresponds to a position estimate. Following its determination of everyposition fix, navigation device 102 may be configured to calculatemeasurement residual R. If R is less than a threshold value, then system100 is deemed to be operating properly. If R is greater or equal to athreshold value, the navigation device 102 may issue an integrity alarm.

FIG. 26 shows the effect of a ranging error on a position solution inaccordance with an embodiment of the invention. Ordinarily, the rangingmeasurements are self consistent. However, should one or more of themeasurements be corrupted and biased, the error could push the outputsolution away from the truth. RAIM is able to detect the error becausethe inconsistency among measurements is highly correlated with theactual position error.

FIG. 27 illustrates how the precision of the system carrier phasecounterbalances occlusion and poor Dilution of Precision (DOP) geometry.In the two-dimensional case, the least squares fit excludes the verticalcomponent of the position error. Advantageously, in one embodiment,system 100 may be implemented with centimeter-level carrier phaseprecision to provide robust navigation during occlusion. As shown, theprocess of FIG. 27 may also use a pre-surveyed altitude map.

FIG. 28 illustrates the use of system 100 to perform navigation usingsignals received directly from LEO satellite 108 and GPS satellites 202in accordance with an embodiment of the invention. FIG. 29 illustrates asimilar implementation of FIG. 28, but with network 316 and rangingsignals 318 added to preclude momentary interruptions in LEO signals 104and GPS signals 106 from affecting the continuity of service.

As previously described, system 100 may be configured to support datauplink 320 from reference stations of reference network 204 tofacilitate navigation performed by navigation devices 102 usingnavigation signals 104B/104C/104D. Data uplink 320 may also be supportedby appropriately-configured navigation devices 102. In this regard, datauplink 320 may also be used to pass any desired data from referencenetwork 204 and/or navigation devices 102 to LEO satellite 108 forsubsequent broadcast as part of communication signal 104A of LEO signal104.

Because GPS Time and UTC are available from a precision timing functionof system 100, it is possible to establish a one-way uplink protocolthat allows data uplink 320 to occur without direct two-waysynchronization. The time and frequency phasing of data uplink 320 canbe pre-positioned to arrive at LEO satellite 108 to exactly match thesatellite's instantaneous carrier phase and frame structure on asymbol-by-symbol basis. Given a suitable multi-use protocol, it ispossible to share the uplink channel among multiple navigation devices102. Such a multi-use protocol may be implemented by time, frequency,code, or any combination thereof. In one embodiment, data uplink 320 maybe configured as a spread spectrum uplink with anti-jamming and lowprobability of intercept and detection (LPI/D) characteristics. Inanother embodiment, low power signals of data uplink 320 may be summedover many symbols to pull an aggregate macro symbol out of the noise andprovide an LPI/D uplink.

FIG. 30 illustrates a generalized frame structure for data bursts 3002of uplink 320 to LEO satellite 108 in accordance with an embodiment ofthe invention. In one embodiment, data uplink 320 may be configured tosupport uplink bursts on approximately 240 channels with 414 bits perburst. For data uplink 320 to be aligned properly on a symbol by symbolbasis, in one embodiment, the frame structure of LEO satellite 108 maybe pre-positioned in a rest state (e.g., no time shift and no frequencyshift relative to a master clock of LEO satellite 108). In anotherembodiment, a reference station of reference network 204 may beconfigured to generate an appropriate synchronization signal for datauplink 320 to LEO satellite 108. The effect of this synchronizationsignal is to pre-align the frame structure for the data symbols in aburst against the UTC or GPS Time reference.

FIG. 31 illustrates a ground infrastructure to synchronize data uplink320 in accordance with an embodiment of the invention. In particular,the ground infrastructure of FIG. 31 includes a reference station ofreference network 204 that may be used to align a payload field 3104 ofeach data burst 3002. In one embodiment, the reference station may beconfigured to not broadcast during the portion of the burst allocated topayload 3104 (this time is reserved for navigation devices 102). In oneembodiment, each of navigation devices 102 may be authorized to uplink asingle symbol within a certain time and frequency slot. In this manner,each symbol (or each orthogonal bit in the QPSK uplink frame structure)is individually addressable by any navigation device 102 that knows itsposition and UTC/GPS Time. Navigation devices 102 may be implemented inaccordance with any appropriate multi-use protocol by which navigationdevices 102 are assigned the bits in the defined fields. For example,under a CDMA protocol, multiple navigation devices 102 may even sharethe same bits.

In various embodiments, data uplink 320 may be implemented with lowpower signals. For example, in one embodiment, uplink 320 may beimplemented using milliwatt-level broadcasts to transmit several bits ofdata per second to LEO satellite 108. If this power is spread over, forexample, a 10 MHz bandwidth, the resulting power flux spectral densityis reasonable for LPI/D applications. Such a spread spectrumimplementation of uplink 320 may also provide antijam protection.

FIG. 32 illustrates an implementation of a low level signal used fordata uplink 320 in accordance with an embodiment of the invention. Inone embodiment, LEO satellite 108 may be configured to receive each bitin a QPSK modulation along with background noise. Because QPSK can besynthesized from two orthogonal binary phase-shift key (BPSK) streams, asimplified BPSK probability distribution (pair of offset Gaussiandistributions) is shown in FIG. 32. Normally, a detector in ademodulator of LEO satellite 108 makes a “1” or “0” (noted here as −1)decision based on a threshold value at zero, and the probability of abit error is calculated by integrating the area under the Gaussian as afunction of SNR.

In one embodiment, the demodulator is treated as a hard limiter. Whenthe SNR is much less than unity, the center Gaussian curve shown in FIG.32 is representative. The presence of a signal (i.e., a data bit) willever so slightly shift the curve from one side to the other, but ingeneral, the output will be swamped by noise. However, by averaging manydiscrete samples together, LEO satellite 108 can detect the emergence ofa signal. Calculations known to those skilled in the art place the lossof a hard limiter at about 2 dB. In other words, but for a 2 dBeffective analog to digital conversion loss, the input signal iscompletely preserved—even if LEO satellite 108 was originallyimplemented as communication satellite. The above approach is notlimited to particular implementations of LEO satellite 108.

In various embodiments, processing of data bits can be performed byreference network 104, navigation device 102, or onboard LEO satellite108. In another embodiment, custom engineered demodulators with amulti-bit RF front end may be used to eliminate the 2 dB hard limiterloss in LEO satellites 108 implemented with analog bent pipeconfigurations.

FIG. 33 illustrates a block diagram of a transmitter 3300 configured tosupport data uplink 320 in accordance with an embodiment of theinvention. In this regard, it will be appreciated that transmitter 3300may be provided as part of a reference station of reference network 204or as part of one or more navigation devices 102. For example, in oneembodiment transmitter 330 may be integrated into a handheld DefenseAdvanced GPS Receiver (DAGR) handheld device, cellular telephonehandset, or any other compact, low-cost device. Advantageously, suchnavigation devices 102 may be configured to permit users of such devicesto send low-latency text or status messages from anywhere in the worldover data uplink 320 for further broadcast over communication signal104A.

As shown in FIG. 33, the position and clock of navigation device 102(e.g., provided by navigation solution 3302), and the position and clockoffset of LEO satellite 108 (e.g., provided by navigation preprocessor1912) are differenced to form an a priori timing advance parameter τ₀used by timing advance calculation block 3308 as shown. In this regard,τ₀ corresponds to the lead time by which the transmission of anindividual data bit, d_(nm), should be advanced to arrive at LEOsatellite 108 at precisely the right time and phasing.

The timing advance parameter then governs the synthesis of the signal inthe baseband processor. The data to be uplinked is encoded and encryptedin block 3304 according to user preference. Data modulator block 3306generates 40% root raised cosine pulses that are modulated by theappropriate data bit, PRN direct sequence code, and channel frequencyoffset provided by PRN generator block 3310 and synthesizer block 3312.Any desired number of channels can be concurrently processed inparallel. The signals are summed, upconverted (in this case by 100 MHz),converted to real form, converted from digital to analog, andupconverted to RF for broadcast as shown by blocks 3316 through 3324 ofFIG. 33.

For compact and low power operation, the baseband component may beimplemented to reside in the modified baseband real estate of a DAGR orcellular handset. In one embodiment, antenna 3324 may also be used forGPS signals in a DAGR or cellular handset. In one embodiment, the powerconsumption and form factor of the data uplink broadcast hardware may beimplemented for handset or compact use. For example, in one embodiment,such transmit hardware may be implemented by a RF2638 chip availablefrom RF Micro Devices that provides 10 dBm of RF output power and draws25 mA at 3V.

FIG. 34 illustrates a block diagram of various components 3400 of LEOsatellite 108 configured to support data uplink 320 in accordance withan embodiment of the invention. In one embodiment, LEO satellite 108 maybe configured to receive data bit impulses through an antenna 3402 and areceiver block 3404, and fill the internal frame structure with theresulting decision, namely +1 or −1. PRN generator block 3406 commandsfrequency hopping on the uplink in a pattern known in advance by bothnavigation device 102 and LEO satellite 108. The direct sequence PRNcode is also applied to the incoming bits by PRN generator block 3408.Waveforms associated with the various macro symbol hypotheses (providedby hypothesis generator block 3410) are mixed with the incoming signaland then processed by a processor 3412 (e.g., in the manner previouslydescribed with regard to processor 718) to provide the resulting datamessage 3414. As with LEO signal 104 also described herein, orthogonalencoding provides excellent bit energy per noise spectral density(Eb/N0) performance for data uplink 320.

Data uplink 320 also contains a built-in ranging signal by virtue of thePRN coding modulation. Optionally, a delay-locked loop (DLL) may beprovided in LEO satellite 108 to estimate the range from navigationdevice 102 to LEO satellite 108. As a result, it is possible to performreverse triangulation and use multiple LEO satellites 108 to passivelytriangulate the position of navigation device 102.

Advantageously, system 100 may be used to provide desired features in avariety of applications. For example, in one embodiment, system 100 maybe implemented to provide rapid, directed rekeying. Using public-privatekey infrastructure techniques with system 100, navigation devices 102may be authenticated using a two-way data link prior to passingencrypted traffic keys over the air. In this manner, positive controlcan be maintained over the specific user, receiver, location, and timeof rekeying.

In another embodiment, system 100 may be implemented to support jointblue force situational awareness. In this regard, navigation devices 102can share position information with other friendly forces nearby, andhazard areas and information on adversary locations can be shared inreal time.

In another embodiment, system 100 may be implemented to supportcommunications navigation and surveillance-air traffic management. Inthis regard, navigation devices 102 may be implemented in aircraft(e.g., in place of the antenna and GPS card in an aircraft's Multi-ModeReceiver (MMR)) to enable Cat III landing, a built-in communicationlink, integrated automatic dependent surveillance, and integratedspace-based air traffic control.

In another embodiment, system 100 may be implemented to support searchand rescue. In this regard, navigation devices 102 may be configured toprovide global E911 features for both military and civil purposes. TheLPI/D characteristics of the military version of data uplink 320 couldqualify a modified DAGR to be employed under hostile conditions.

In another embodiment, system 100 may be implemented to support enrouteretargeting. In this regard, guided munitions may be commanded orretargeted in real time using commands issued by a modified DAGR.

In another embodiment, system 100 may be implemented to support battledamage assessment. In this regard, information gathered in human orsensor form, including position information, can be quickly aggregatedvia data uplink 320. In another embodiment, system 100 may beimplemented to support weather information correlated by position can beaggregated in real time.

In another embodiment, system 100 may be implemented to permit a networkof navigation devices 102 to aggregate measurements of jammer power oruse time or frequency characteristics in a jammer to triangulate theirexact locations.

In another embodiment, system 100 may be implemented to support spotbeam control. In this regard, an envelope of authority to control spotbeam power for antijam purposes may be delegated to navigation devices102. For example, if jamming is experienced, navigation devices 102 maybe configured to request a real-time increase in the broadcast power ofLEO signal 104. Such an implementation could be made available tomilitary or civil safety of life users, with the envelope of authoritydetermined by government policy.

In another embodiment, system 100 may be implemented to support globalcellular text messaging. For example, data uplink 320 capability may beprovided in navigation device 102 (e.g., a modified DAGR or cellulartelephone handset) to permit text messages to be sent to and from anylocation worldwide.

Embodiments described above illustrate but do not limit the invention.It should also be understood that numerous modifications and variationsare possible in accordance with the principles of the present invention.Accordingly, the scope of the invention is defined only by the followingclaims.

1. A method of performing navigation, the method comprising: receiving alow earth orbit (LEO) signal from a LEO satellite; decoding a navigationsignal from the LEO signal; receiving first and second ranging signalsfrom first and second ranging sources, respectively; determiningcalibration information associated with the first and second rangingsources; and calculating a position using the navigation signal, thefirst and second ranging signals, and the calibration information. 2.The method of claim 1, wherein the LEO signal comprises a communicationsignal and the navigation signal, wherein the LEO satellite is acommunication satellite configured to provide the LEO signal.
 3. Themethod of claim 2, wherein the LEO satellite is selected from the groupconsisting of an Iridium satellite and a Globalstar satellite.
 4. Themethod of claim 2, wherein the navigation signal comprises a pseudorandom noise (PRN) signal encoded in a plurality of channels of the LEOsignal.
 5. The method of claim 1, wherein the navigation signal isselected from the group consisting of a military navigation signal, acommercial navigation signal, and a civilian navigation signal.
 6. Themethod of claim 1, wherein at least one of the ranging signals isselected from the group consisting of a cellular telephone signal, atelevision signal, and a global positioning system (GPS) signal.
 7. Themethod of claim 1, wherein the calibration information is encoded in theLEO signal, wherein the determining comprises decoding the calibrationinformation from the LEO signal.
 8. The method of claim 1, wherein thecalibration information comprises code timing, carrier phase, data bits,and symbol phase.
 9. The method of claim 1, wherein the determiningcomprises receiving the calibration information from a reference stationin communication with the ranging sources.
 10. The method of claim 1,wherein the determining comprises receiving the calibration informationthrough a cellular network.
 11. The method of claim 1, furthercomprising: receiving a replica of the navigation signal through acellular network; and calculating the position using the replica of thenavigation signal, the first and second ranging signals, and thecalibration information.
 12. The method of claim 1, wherein the methodis performed by a device selected from the group consisting of ahandheld navigation device, a vehicle-based navigation device, and anaircraft-based navigation device.
 13. The method of claim 1, wherein thedecoding comprises: selecting a plurality of channels of the LEO signalcarrying the navigation signal, wherein the navigation signal comprisesnavigation data encoded in a navigation carrier signal, wherein thenavigation carrier signal is distributed over the channels by a pseudorandom noise (PRN) encoding; despreading the navigation signal from thechannels; and demodulating the navigation data and the navigationcarrier from the navigation signal.
 14. The method of claim 1, whereinthe decoding comprises decrypting the navigation signal using adistributed encryption key associated with an area of operations. 15.The method of claim 1, wherein at least one of the ranging signals is asingle-frequency global positioning system (GPS) L1 signal.
 16. Themethod of claim 1, wherein performance of the method is conditioned onpossession of an encryption key by a navigation device.
 17. A navigationdevice comprising: an antenna adapted to receive a low earth orbit (LEO)signal from a LEO satellite and receive first and second ranging signalsfrom first and second ranging sources, respectively; a receiverprocessor adapted to downconvert the LEO signal for further processing;and a navigation processor adapted to decode a navigation signal fromthe LEO signal, and adapted to calculate a position of the navigationdevice using the navigation signal, the first and second rangingsignals, and calibration information associated with the first andsecond ranging sources.
 18. The navigation device of claim 17, whereinthe LEO signal comprises a communication signal and the navigationsignal, wherein the LEO satellite is a communication satelliteconfigured to provide the LEO signal.
 19. The navigation device of claim18, wherein the LEO satellite is selected from the group consisting ofan Iridium satellite and a Globalstar satellite.
 20. The navigationdevice of claim 18, wherein the navigation signal comprises a pseudorandom noise (PRN) signal encoded in a plurality of channels of the LEOsignal.
 21. The navigation device of claim 17, wherein the navigationsignal is selected from the group consisting of a military navigationsignal, a commercial navigation signal, and a civilian navigationsignal.
 22. The navigation device of claim 17, wherein at least one ofthe ranging signals is selected from the group consisting of a cellulartelephone signal, a television signal, and a global positioning system(GPS) signal.
 23. The navigation device of claim 17, wherein thecalibration information is encoded in the LEO signal, wherein thenavigation processor is adapted to decode the calibration informationfrom the LEO signal.
 24. The navigation device of claim 17, wherein thecalibration information comprises code timing, carrier phase, data bits,and symbol phase.
 25. The navigation device of claim 17, wherein thenavigation device is adapted to receive the calibration information froma reference station in communication with the ranging sources.
 26. Thenavigation device of claim 17, wherein the calibration information isreceived through a cellular network.
 27. The navigation device of claim17, wherein the antenna is adapted to receive a replica of thenavigation signal through a cellular network, wherein the navigationprocessor is adapted to calculate the position using the replica of thenavigation signal, the first and second ranging signals, and thecalibration information.
 28. The navigation device of claim 17, whereinthe navigation device is selected from the group consisting of ahandheld navigation device, a vehicle-based navigation device, and anaircraft-based navigation device.
 29. The navigation device of claim 17,wherein the navigation processor is adapted to: select a plurality ofchannels of the LEO signal carrying the navigation signal, wherein thenavigation signal comprises navigation data encoded in a navigationcarrier signal, wherein the navigation carrier signal is distributedover the channels by a pseudo random noise (PRN) encoding; despread thenavigation signal from the channels; and demodulate the navigation dataand the navigation carrier from the navigation signal.
 30. Thenavigation device of claim 17, wherein the navigation processor isadapted to decrypt the navigation signal using a distributed encryptionkey associated with an area of operations.
 31. The navigation device ofclaim 17, wherein at least one of the ranging signals is asingle-frequency global positioning system (GPS) L1 signal.
 32. Anavigation device comprising: means for receiving a low earth orbit(LEO) signal from a LEO satellite; means for decoding a navigationsignal from the LEO signal; means for receiving first and second rangingsignals from first and second ranging sources, respectively; means fordetermining calibration information associated with the first and secondranging sources; and means for calculating a position using thenavigation signal, the first and second ranging signals, and thecalibration information.
 33. The navigation device of claim 32, whereinthe navigation device is part of a navigation and communication systemcomprising the LEO satellite, a reference network, and the first andsecond ranging sources.
 34. The navigation device of claim 32, means forestimating ionosphere effects using a single-frequency globalpositioning system (GPS) L1 signal.
 35. The navigation device of claim32, further comprising means for providing three-dimensional autolandguidance for an aircraft using the navigation signal and the firstsignal, wherein the first ranging source is a satellite.
 36. Thenavigation device of claim 32, further comprising means for providingvertical autoland guidance using the navigation signal.
 37. Thenavigation device of claim 32, further comprising means for providingtwo-dimensional vehicle guidance using an altitude map and thenavigation signal.
 38. The navigation device of claim 32, furthercomprising means for implementing Receiver Autonomous IntegrityMonitoring (RAIM).